Coal is the world's most abundant fossil fuel. However, coal has three major drawbacks: (1) Coal is a solid and is less easily handled and transported than fluidic or gaseous materials; (2) Coal contains compounds which, on burning, produce the pollutants associated with acid rain; and (3) Coal is not a uniform fuel product, varying in characteristics from region to region and from mine to mine.
In fossil fuels, the ratio of hydrogen atoms to carbon atoms is most important in determining the heating value per unit weight. The higher the hydrogen content, the more liquid (or gaseous) the fuel, and the greater its heat value. Natural gas, or methane, has a hydrogen-to-carbon ratio of 4 to 1 (this is the maximum); gasoline has a ratio of almost 2.2 to 1; petroleum crude about 2.0 to 1; shale oil about 1.5 to 1; and coal about 1 to 1.
The lignites, peats, and lower calorific value sub-bituminous coals have not had an economic use except in the vicinity of the mine site, for example, mine mouth power generation facilities. This is due primarily to the cost of shipping a lower Btu, higher moisture product as well as to the danger of spontaneous combustion because of the high content of volatile matter which is characteristic of such coals.
The risk of spontaneous combustion of low-rank coals is increased by dehydration, even by non-evaporation methods. Therefore, in order to secure stability of the dehydrated coal in storage and transportation, it has been necessary to cover the coal with an atmosphere of inert gas such as nitrogen or combustion product gas, or to coat it with crude oil so as not to reduce its efficiency as a fuel. However, these methods are not very economical.
Waste coal has somewhat different inherent problems from those of the low-rank coals. Waste coal is sometimes referred to as a "non-compliance coal" because it is too high in sulfur per unit heat value to burn in compliance with the United States Environmental Protection Agency (EPA) standards. Other waste coal is too low in heat value to be transported economically.
The inefficient and expensive handling, transportation and storage of all types of coal (primarily because it is a solid material) prevent coal from being an exportable commodity and cause the conversion of oil-fired systems to coal to be economically unattractive. Liquids are much more easily handled, transported, stored and fired into boilers.
Besides being difficult to transport, coal is a heterogeneous fuel, i.e, coal from different reserves has a wide range of characteristics. Coal from one region (or even of a particular mine) cannot be efficiently combusted in boilers designed for coal from another source. Boilers and pollution control equipment must either be tailored to a specific coal or configured to burn a wide variety of material with a resultant loss in efficiency.
The non-uniformity and transportation problems are compounded by the presence of combustion pollutants in coal such as sulfur and nitrogen compounds which are thought to cause acid rain. The sulfur compounds are of two types: organic and inorganic (pyritic). The fuel bound nitrogen, i.e., organic nitrogen in the coal, combusts to form NO.sub.x. Further, because of the non-uniformity of coal, it combusts with "hot spots" which result in some of the nitrogen in the combustive air (air is 75% nitrogen by weight) being oxidized to produce NO.sub.x. This so-called "thermal NO.sub.x " has heretofore been reduced only by boiler modification systems or expensive catalytic reduction systems.
Raw coal cleaning has heretofore been available to remove inorganic ash and sulfur; however, traditional cleaning has not been able to remove the organic nitrogen and organic sulfur compounds which, upon combustion, produce the SO.sub.x and NO.sub.x pollutants. Heretofore precombustion technologies such as fluidized bed boilers, which require limestone as an SO.sub.x reactant, and post-combustion technologies such as scrubbers or NO.sub.x selective catalytic convertors have been the main focus in seeking to alleviate these pollution problems. These devices clean the combustion and flue gas rather than the fuel and are tremendously expensive from both capital and operating standpoints, adding to the cost of power. This added power cost not only increases the cost of domestically produced goods, but also ultimately diminishing this nation's competitiveness with foreign goods. Moreover, operation of post-combustion pollution control equipment draws on the power generated in the plant, reducing saleable plant output. This inefficiency results in higher production of CO.sub.2 per unit of power available for sale. CO.sub.2 production has been linked with the "greenhouse" effect, i.e. the warming of the earth's atmosphere.
It would, therefore, be advantageous to clean up the coal by removing the organic nitrogen (fuel nitrogen) as well as both the organic and the pyritic sulfur while providing a uniform, highly reactive fuel which burns at a lower temperature, thereby reducing the production of thermal NO.sub.x.
In order to overcome some of the inherent problems with coal as a solid fuel, various methods have been proposed for utilizing coal to produce synthetic liquid or gaseous fuels. These liquefaction or "synfuel" processes are capital intensive and require a great deal of externally supplied water and hydrogen, i.e., hydrogen and water provided from other than the coal feedstock. These processes are also energy intensive in that most carbon atoms in the coal matrix are converted to hydrocarbons, i.e., no pure carbon. This differs markedly from merely "rearranging" existing hydrogen in the coal molecule as in hydrodisproportionation, to hydrogenate certain carbon atoms at the expense of other carbon atoms.
Coal pyrolysis is a well-known process whereby coal is thermally volatilized by heating the coal out of contact with air. Different pyrolysis products may be produced by varying the conditions of temperature, pressure, atmosphere, and/or material feed. Traditional pyrolysis has produced very heavy hydrocarbon tars and carbon (char), with the liberation of hydrogen.
In prior art pyrolysis, as shown in FIG. 2, the pulverized coal is heated relatively slowly at low heating rates and long residence times such that the coal molecule undergoes a decomposition at reaction rate `k.sub.1 ` to yield "decomposition" products, primarily free radical hydrocarbon fragments. These "decomposition" products undergo a recomposition or "condensation" reaction at reaction rate `k.sub.2 `, producing char and dehydrogenated hydrocarbons and liberating hydrogen. The decomposition reaction is not desirable in a refining process because it liberates valuable hydrogen instead of utilizing it to upgrade the hydrocarbon products. As shown in FIG. 2, when k.sub.1 and k.sub.2 overlap, the dehydrogenation of the decomposition product, i.e., condensation reaction, is predominant. It is believed that when the coal is heated slowly, the decomposition reaction takes place within the coal particle where there is little hydrogen present to effect the hydrogenation reaction. Thus, the decomposed molecular fragments condense, which results in the production of heavy tar-like liquids of very limited utility.
Other prior art processes for treatment of bituminous and subbituminous coals of various ranks attempted to hydrogenate decomposition products by adding external hydrogen. These hydropyrolysis processes have been carried out in both the liquid and gaseous phases with the most economical processes taking place under milder conditions. However, these processes have had only limited success in producing economical products. As in pyrolysis, if the heating rates are not rapid, the decomposition material remains inside the coal particles and cannot be hydrogenated by external hydrogen. In order to promote hydrogenation, more stringent reaction conditions were employed, reducing the economic viability. Examples of such processes are disclosed in U.S. Pat. Nos. 4,704,134; 4,702,747; and 4,475,924. In such processes, coal is heated in the presence of hydrogen or a hydrogen donating material to produce a carbonaceous component called char and various hydrocarbon-containing oil and gas components. Many hydropyrolysis processes employ externally generated additional hydrogen which substantially increases the processing cost and effectively makes the process a "liquefaction" process.
A particular type of coal hydropyrolysis, flash hydropyrolysis, is characterized by a very short reactor residence time of the coal. Short residence time (SRT) processes are advantageous in that the capital costs are reduced because the feedstock throughput is so high. In SRT processes, high quality heat is required to effect the transformation of coal to char, liquids and gases.
In many processes, hydrogen is oxidized within the reactor to gain the high quality heat. However, the oxidation of hydrogen in the reactor not only creates water but also reduces the amount of hydrogen available to hydrogenate hydrocarbons to produce higher quality fuels. Thus, in prior art processes, either external hydrogen is required or the product is degraded because valuable hydrogen is converted to water.
The prior art methods of deriving hydrogen for hydropyrolysis are either by: (1) purchasing or generating external hydrogen, which is very expensive; (2) steam-methane reforming followed by shift conversion and CO.sub.2 removal as disclosed in a paper by J. J. Potter of Union Carbide; or (3) char gasification with oxygen and steam followed by shift conversion and CO.sub.2 removal as disclosed in a paper by William J. Peterson of Cities Service Research and Development Company.
All three of these hydrogen production methods are expensive, and a high temperature heat source, such as direct O.sub.2 injection into the hydropyrolysis reactor, is still required to heat and devolatilize the coal. In the prior art processes, either carbon (char) is gasified by partial oxidation, such as in a Texaco gasifier (U.S. Pat. No. 4,491,456 to Schlinger and U.S. Pat. No. 4,490,156 to Marion et al.), or oxygen is injected directly into the reactor. One such system is disclosed in U.S. Pat. No. 4,415,431 (1983) of Matyas et al. When oxygen is injected directly into the reactor, it preferentially combines with hydrogen to form heat and water. Although this reactor gives high-quality heat, it uses up hydrogen which is then unavailable to upgrade the hydrocarbons. This also produces water that has to be removed from the reactor product stream and/or floods the reactor. Additionally, the slate of hydrocarbon co-products is limited.
Flash hydropyrolysis has additional drawbacks in that the higher heating rates associated with short residence times tend to thermally hydrocrack and gasify the material. This gasification reduces liquid yield and available hydrogen.
Thus, it would be advantageous to have a means for producing: (1) a high yield of liquid hydrocarbons, (2) high quality heat for volatilization, (3) hydrogen, and (4) other reducing gases prior to the reaction zone without producing large quantities of water and without using up valuable hydrogen.
In U.S. Pat. Nos. 4,671,800 and 4,658,936, it is disclosed that coal can be subjected to pyrolysis or hydropyrolysis under certain conditions to produce a particulate char, gas and a liquid organic fraction. The liquid organic fraction is rich in hydrocarbons, is combustible, can be beneficiated and can serve as a liquid phase for a carbonaceous fluidic fuel system. The co-product distribution, for example hydrocarbon fractions such as BTX and naphtha, and the viscosity, pumpability and stability of the fuel when the char is admixed with the liquid organic fraction are a function of process and reaction parameters. The rheology of the fuel system is a function of solids loading, sizing, surfactants, additives, and oil viscosity.
The economic feasibility of producing the fluidic fuel is predicated on the method of volatilizing the coal to produce the slurry and a slate of value-added co-products. The economics of transporting the fluidic fuel is predicated upon the rheology of the fuel system.
Common volatilization reactors include the fluidized bed reactor which uses a vertical upward flow of reactant gases at a sufficient velocity to overcome the gravitational forces on the carbonaceous particles, thereby causing suspension of the particles in a gaseous medium. The fluidized bed reactor is characterized by particles subjected to longer reaction residence times to obtain conversion into liquid and gaseous hydrocarbons. Thus, this type of reactor is not very conducive to short residence time (SRT) processing and may produce a large quantity of polymerized (tar-like) hydrocarbon co-products.
Another common reactor is the entrained flow reactor which utilizes a high-velocity stream of reactant gases to impinge upon and carry the carbonaceous particles through the reactor vessel. Entrained flow reactors are characterized by smaller volumes of particles and shorter exposure times to the high-temperature gases. Thus, these reactors are useful for SRT-type systems.
In one prior art two-stage entrained flow reactor, a first stage is used to react carbonaceous char with a gaseous stream of oxygen and steam to produce hydrogen, oxides of carbon, and water. These products continue into the second stage where volatile-containing carbonaceous material is fed into the stream. The carbonaceous feed reacts with the first-stage gas stream to produce liquid and gaseous hydrocarbons, including large amounts of methane gas and char.
Prior art two-stage processes for the gasification of coal to produce primarily gaseous hydrocarbons include U.S Pat. Nos. 4,278,445 to Stickler; 4,278,446 to Von Rosenberg, Jr.; and 3,844,733 to Donath. U.S. Pat. No. 4,415,431 issued to Matyas et al. shows use of char as a carbonaceous material to be mixed with oxygen and steam in a first-stage gasification zone to produce a synthesis gas. Synthesis gas, along with additional carbonaceous material, is then reacted in a second-stage hydropyrolysis zone wherein the additional carbonaceous material is coal to be hydropyrolyzed.
U.S. Pat. No. 3,960,700 to Rosen describes a process for exposing coal to high heat for short periods of time to maximize the production of desirable hydrocarbons.
One method of terminating the volatilization reaction is by quenching the products either directly with a liquid or gas, or by use of a mechanical heat exchanger. In some cases, product gases or product oil are used. Many reactors, including those for gasification have employed a quench to terminate the volatilization reaction and prevent polymerizing of unsaturated hydrocarbons and/or gasification of hydrocarbon products. Some have employed intricate mechanical heat-exchange quenches to attempt to capture the heat of reaction. One such quench scheme is shown in U.S. Pat. No. 4,597,776 issued to Ullman et al. The problem with these quench schemes is that they introduce mechanical apparatus into the reaction zone. This can cause tar and char accumulation on the heat-exchanger devices, thereby fouling the heat exchanger.
Thus, if the coal has a hydrogen-to-carbon ratio of 1, and if the hydrogens on half the carbons could be transferred or "rearranged" to the other half of the carbons, then the result would be half the carbons with 0 hydrogens and half with 2 hydrogens. The first portion of carbons (with 0 hydrogens) is char; the second portion of carbons (with 2 hydrogens) is a liquid product similar to a petroleum fuel oil. If this could be accomplished using only hydrogen inherent in the coal, i.e., no external hydrogen source, then the coal could be refined in the same economical manner as petroleum, yielding a slate of refined hydrocarbon products and char.
It would be highly advantageous to have a fuel system which is easily and efficiently prepared solely from coal using no external water and producing a slate of clean burning co-products including benzene, toluene, xylene (BTX); ammonia; sulfur; naphtha; fuel oil; and methanol as well as a clean burning boiler fuel which is: (1) transportable using existing pipeline, tanker car and tankership systems; (2) burnable either directly as a substitute for oil in existing oil-fired combustion systems with little or no equipment modification, or separable at the destination to provide a liquid hydrocarbon fuel or feedstock and a burnable char; (3) a uniform product regardless of the region from which the coal is obtained; (4) high in BTU content per unit weight and volume; (5) high in solid loading and stability; (6) low in ash, sulfur and nitrogen; and (7) free of other polluting effluents which would have to be disposed of at the production site or at the destination.
Further, it would be highly advantageous to have a system for refining coal wherein short residence times and internally generated hydrogen are used in mild conditions in the presence of a catalyst to efficiently produce larger quantities of hydrocarbon liquids without excess gasification of such products. In such a system, inherent hydrogen in the coal could be conserved to increase the co-product value and minimize the process operating costs.
In U.S. patent application Ser. No. 277,603, now U.S. Pat. No. 4,938,782 issued Jul. 3, 1990, it is disclosed that coal can be refined in the presence of internally generated hydrogen to effect high oil yields, without attendant gas production and heavy tar formation. It has now been unexpectedly discovered that the production of hydrocarbon liquids can be greatly increased by use of a catalyst which is introduced into the reactor with the feedstock and/or contained on a medium within the reactor. Surprisingly, the carbon conversion of the coal feedstock and the hydrocarbon liquids is greatly increased by use of a catalyst.